Examples of such power-electronic circuits are the modular multilevel convertor M2C (see U.S. Pat. No. 7,269,037; DE 101 03 031) or the modular multilevel parallel-serial convertor M2SPC (see WO 2012072197; DE 102010052934; WO 2012072168; WO 2012072197; EP 20110179321; DE 20101052934; WO 2013017186; DE 102011108920). In addition, what are referred to as switched-capacitor circuits meet the necessary preconditions for use together with the present invention, i.e. the use of a plurality of energy stores whose electrical connections to one another can be varied dynamically in order to exchange energy between the energy stores and/or to control electrical parameters (in particular the current and/or voltage) at terminals for external systems (for example energy grids, electrical consumers or sources). The invention is not necessarily tied to a specific convertor or type of module here.
The term module is used below to refer to any unit in a power-electronic circuit which has a plurality of electrical energy stores and respectively contains an energy store and at least one electronic switch which contributes to dynamically changing the electrical connection of the at least one energy store of the module to at least one energy store of another module. This changing of the electrical connection of the at least one energy store to at least one energy store of another module can for example also refer to the changeover from a virtual series connection of these energy stores to bypassing of one or more of these energy stores by conducting the current past the energy store without charging or to discharging it (referred to as the bypass state). Therefore, the term also refers to switched capacitor circuits. In the text which follows, all these circuits are referred to as converters. Different modules can be fabricated independently of one another, for example, on separate electronic units, for example electronic circuit boards, in order to use cost reduction effects in production owing to relatively high production numbers of similar units, and/or to use maintenance through the possibility of easily replacing potentially closed-off individual modules.
The switched state of a module refers to the way in which the switch or switches of the module are activated or deactivated in order to connect at least one electrical energy store of the module in an electrically conductive fashion to at least one electrical energy store of at least one other module in a different way, referred to as the connectivity, or explicitly not to connect it (open circuit or disconnected connection), in such a way that a plurality of modules together generate an electrical voltage. Examples of possible connectivities of electrical energy stores are, for example, a parallel connection and series connection, combinations of electrical energy stores as well as energy stores which are unconnected or connected only to one contact.
Exemplary elementary circuits from the prior art which can be a basis for the invention are the modular multilevel converter M2C (U.S. Pat. No. 7,269,037; DE 101 03 031), the modular multilevel converter M2SPC (WO 2012072197; DE 102010052934; WO 2012072168; WO 2012072197; EP 20110179321; DE 20101052934; WO 2013017186; DE 102011108920) as well as most of what are referred to as switched capacitor circuits (see, for example, [M. D. Seeman, S. R. Sanders (2008). Analysis and optimization of switched-capacitor dc-dc converters. IEEE Transactions on Power Conversion, 23(2): 841(ff)]), Modular multilevel converters such as the M2C or the M2SPC are based on the interconnection of modules with a generally significantly lower voltage than the entire output voltage of the modular converter which is formed therefrom. Depending on the type of converter, the corresponding module connection breaks up the overall voltage, overall current and necessary switching rate of the individual electronic switchers into small partial units, in contrast with a traditional converter with fewer than four levels (for example H-bridges). In the M2C, individual modules are composed, for example, of an energy store, for example a capacitor, and a plurality of electronic switchers which are arranged either in a half-bridge or in a full bridge. Each M2C module is therefore a dipole which can, for example, be connected in series and/or parallel with other modules, in order to form what is referred to as a macrotopology, i.e. a combination of modules for performing electrical converter tasks.
In this context, there are further derivations and developments such as those described, for example, in U.S. Ser. No. 13/990,463; U.S. Ser. No. 14/235,812; DE 10 2010 008978; DE 10 2009 057288; U.S. Pat. No. 3,581,212.
Such modules, referred to as microtopologles, can be linked to one another in different ways in what are referred to as macrotopologies. The macrotopology which occurs most frequently is the Marquardt topology (see U.S. Pat. No. 7,269,037; DE 102010052934), which is illustrated for the M2C and the M2SPC in FIG. 1. In this macrotopology, a plurality of phase modules or phase units, which are composed themselves of converter arms (the interconnection of at least two modules) exist which are connected to one another at the ends. In addition, there are numerous further variants for the connection of modules, in the simplest cases a simple chain of modules.
In virtually all macrotopologies, the smallest unit is what is referred to as the converter arm. It constitutes a line of at least two similar modules which are connected to one another. The similarity is already given by the fact that the adjacent modules have in common at least two common states (for example serial positive and bypass) and can be used with dynamic changes between them during operation.
Typical examples of switched capacitor circuits are voltage pumps in which electrical energy stores, for example capacitors, can change their connectivity from a (partial) series circuit to a (partial) parallel circuit and can therefore change their generated voltage. An example of this is the Marx converter, often referred to as a Marx generator [see Erwin Marx (1925). Erzeugung von verschiedenen Hochspannungsarten zu Versuchs-und Prüfzwecken [Generation of various types of high voltage for experimental and testing purposes]. Elektrotech. Zeitschrift (Electroengineering periodical)], and refinements thereof [for example J. Rodriguez, S. Leeb (2006). A multilevel inverter topology for inductively coupled power transfer. IEEE Transactions on Power Electronics, 21(6): 1607ff.; F. Peng, W. Qiang, D. Cao (2010). Recent advances in multilevel converter/inverter topologies and applications. International Power Electronics Conference (IPEC), 492ff.]. Such circuits are already in existence as DC voltage converters, inverters and converters. By suitably dividing the circuit into parts which each contain at least one energy store and at least one electrical switch, such circuits can be modularized. This provides advantages in terms of industrial production. The modular multilevel converter M2SPC mentioned above can be regarded as a combination or hybrid of traditional multilevel converters and switched-capacitor converters.
A central problem during the operation of many such circuits which are composed of a number of electrical stores whose electrical connection to one another (connectivity) can be changed dynamically during operation—for example can be changed dynamically from an electrical series circuit to an electrical parallel circuit in order in the process to reduce the overall voltage, the overall internal resistance and the current per module—resides in the balancing of the energy stored in each module. As a rule, the stores are electrical capacitors and/or batteries. The latter must not undershoot or exceed generally determined voltage limits here and should advantageously meet simultaneously determined further conditions relating to their electrical conditions (in particular voltage and current), in order to avoid aging too quickly. Because of the constant exchange of energy of each module with its neighbors as well as with external systems (for example the power grid, a motor or a generator) via all the terminals, the dynamically changeable connectivity serves, on the one hand, to selectively influence the energy content of each module by charging or discharging. On the other hand, precise knowledge of the current and/or previous electrical conditions of the respective modules is necessary for selected balancing and symmetrizing of the energy contents of individual modules or the control of the electrical conditions (in particular voltage and current). On this data base, one or more control units make decisions about the connectivity to be used in the future in order to adapt the electrical conditions such as the voltage, current and energy content of the individual modules with their integrated energy stores to predefined or determined target values, usually by means of selective adjustment.
In conventional solutions, for this purpose a complex measurement of the electrical conditions (in particular voltage and current) of the individual modules is usually carried out and transmitted to one or more control units.
In order to ensure the stringent necessary safety conditions and stability conditions, this measurement is usually carried out with several kilohertz of sampling rate. When significantly more than a hundred modules are used in many industrial implementations of the modular multilevel converter M2C, measurement data of the voltage and current which is sampled with high resolution, with several megahertz of data rate, is produced and has to be transmitted to the central unit and processed there. Since the individual measurement points in the modules are not related to a common reference potential, and the ground potential in each module also depends on the connectivity of all the other modules, the recording of the measured values and their transmission take place in an electrically insulated fashion (with separated potentials). On the one hand, this increases the complexity and susceptibility of the system to faults, and on the other hand this fact gives rise to high costs because the transmission has to make use of expensive components, for example high-speed optical transmission.
In the relevant literature, a number of approaches for remedying this problem have been discussed. L. Änquist et al. (2011) [L. Änquist, A. Antonopoulos, D. Siemaszko, K. Ilves, M. Vasiladiotis, H.-P. Nee (2011). Open-loop control of modular multilevel converters using estimation of stored energy. IEEE Transactions on Industry Applications. 47(6): 2516ff.] describe, for example, a method for estimating the energy content in each module on the basis of the measured current in an arm. Compared to the currently prevailing approach of balancing, symmetrizing and adjusting the energy content on the basis of measured values by means of each module, this approach eliminates the need for many of the necessary sensors, reduces the quantity of data to be processed by several orders of magnitude and can make available shorter adjustment reaction times owing to the elimination of the delay in the recording of the measured values and the multiplexing of data. The energy content of the energy stores in each module is estimated on the basis of the respective module current. The module current is in turn itself an estimated value which is derived from the measured arm current and the electrical switching state of the module (i.e. the connectivity, in this case the series circuit or bypass). The determination of the energy content of the respective module memories is carried out by determining the net charge over time by means of the respective module current.
However, this approach to a solution entails a substantial problem. The estimation of the individual module energy contents is based on the accumulation of the current and consequently contains a time integral over a current measured value. This integral is here not a short-term integral for averaging noise (for example in the form of a low-pass filter) but instead the calculation of a charge. In particular, current measurements are always to be considered to be subject to faults. In particular, slight measuring offsets, that is to say deviations of the measured zero point from the actual one, are present in virtually all current measurement systems. In addition, practical measurement systems are themselves often integrating or differentiating (for example typical Hall sensor-based current clamp meters or Rogowski coils) and therefore are not at all able to measure constant components.
As result of the integral over the current during the determination of the energy contents of the energy stores (also referred to as state of charge, SOC for short) small measuring offsets over time are amplified to a random degree. As result of the integral over time, even very small permanent offsets such as, for example, deviations caused by an analog-to-digital converter can lead to a substantial overestimation or underestimation of the actual energy content in the individual modules. Compensation or detection of the fault in a control unit is possible only given complete knowledge of the long-term energy take up at all the interfaces (terminals) with external systems. For example, Änquist et al. (2011) assumes perfectly harmonic currents which are offset-free, as result of which it would be possible to discover a potential offset in the measured values. However, since a real converter can control the currents with respect to its external terminals and in its arms itself and in the case of incorrect estimation can very easily take up more (or less) energy net than it outputs, such an assumption is not helpful. In addition, the described solution is only for very special converters and operating conditions. For example, an DC/AC converter is assumed. The modules used can be exclusively M2C modules with their very limited module states (serial circuit and bypass). The approach fails in the M2SPC modules with their parallel connectivity or else other module topologies. In addition, the approach to a solution must, for its functioning, dispense with all real problems of power-electronic circuits. These include, in particular, internal resistance switches of all the components and leakage currents in electronics and storage elements, for example capacitors and batteries. Because of the high fabrication variation of these two properties, even in components of the same series, they can, however, not be ignored, particularly in an integrating approach.
In commercial converter systems based on modular multilevel converters, which can often cost several hundred million dollars and have to be designed for uninterrupted use over several years (consequently the integrating time in the above approach to a solution from the state of the research), a systematic hazard resulting from a deficient control system with systematic faults which almost indisputably bring about destruction of the system, and under certain circumstances, fires, is therefore not advantageous.
Alternative, more stable technical approaches to the solution of the problem of the necessary precise knowledge of the electrical parameters in any model have not been presented until now. On the one hand, the practical problem when using current measured values which are integrated over time has not been sufficiently acknowledged or tested in realistic test environments. On the other hand, in specialist circles another solution to this technical problem is considered to be difficult and has not been satisfactorily solved by industrial development departments either. For this reason, despite the extremely high costs, measurement and monitoring of each individual energy store with respect to the operation of such converters continues to be the only approach used industrially.